Twenty new Rett syndrome (RS) patients and 15 patients with RS, evaluated at least 2 years previously were admitted to the PCRU per year. The purpose of the study is to determine if RS is a progressive neurodegenerative disorder or a developmental disorder of early infancy followed by a static course later. To this end, patients are compared at different time points, with a minimum of 2 years between evaluations. Neurological, developmental, speech and physical therapy assessments are compared. For more objective evidence, MRI volumetric assessment, EEG and anthropometric measures are used. Preliminary studies do not indicate evidence for a progressive disease. Positron Emission Tomography (PET) studies of the brain surprisingly show low-normal levels of D2 dopamine receptors (post-synaptic) and dopamine uptake sites (pre-synaptic), suggest that there is a compensatory upregulation of the dopaminergic system, and in part explain the ineffectiveness of L-dopa to relieve the extrapyramidal symptoms in RS.